1. Field of the Invention
This invention relates to optical systems for forming distortionless images having microscopically fine geometries and more particularly relates to a unit magnification catadioptric imaging system having micron resolution capability over a broad frequency spectrum for use in fabricating microcircuits.
2. Description of the Prior Art
In fabricating microcircuits a silicon slice or wafer is coated with a photoresist, exposed to a light pattern formed by one or more masks and subjected to processing steps. This sequence is repeated many times to build up microcircuits on the wafer. A wafer is typically 75 or 100 mm diameter and has many hundreds or thousands of individual identical circuits formed on it having lines of one to two microns in width. Successive masks need to be positioned with micron precision. Individual circuit masks are generally produced by photographic reduction from larger master drawings or masks. Masks containing an array of identical circuits are produced from a drawing or mask containing only one such circuit with a step-and-repeat camera system.
The final transfer of the pattern to the wafer is typically done by contact printing. Contact masks are inherently difficult to register or align properly and furthermore readily become damaged or worn from the contact. Thus, projection printing is better for the final transfer if the masks and optical system have sufficient resolution capability. If projection printing is used for the final transfer, it is also possible to perform the step-and-repeat operation at this stage. However, there are stringent optical requirements. For good linewidth production, the minimum modulation required is about 60%. For one micron lines this requires a numerical aperture of about 0.30 and an optical system which is aberration corrected so well that it is diffraction limited. It is furthermore desirable to have such correction over a wide spectral range so that alignment may be done at a visual frequency to which the photoresist is insensitive while the exposure may be done at a much shorter wavelength, typically in the near ultraviolet region, where sharper edges and corners are obtained in the pattern. Since it is difficult to mechanically assure that each successive mask and wafer alignment will be at exactly the predetermined focal settings, it is also desirable that the projection system should be telecentric in both image and object space, which means it should be a unit magnification telescope.
Optically produced masks already have rounded edges and corners at a linewidth of one micron due to the loss of higher harmonics. This deficiency can be compensated for in the optical system by increasing the numerical aperture (N.A.) to perhaps 0.70, but then the field covered as a result would be so small that the system would be impractical. Electron beam produced masks of better quality have sufficiently sharp edges and corners and will be preferred in this application.
Conventional dioptric systems are capable of providing micron resolution but only at a single wavelength and at substantial complexity if a larger field is desired. A typical 5.times. high performance lens system, for example, has about 11 elements of 60 to 90 mm diameter for a 10.times.10 mm field. A similar lens construction for unity magnification would be totally impractical since the lens system would now have to provide the same NA on both the image and the object sides leading to about 20 to 22 glass elements. A further disadvantage of conventional dioptric systems is that variations with frequency of Seidel aberrations and higher order aberrations cannot be corrected, which results in having a substantial secondary spectrum.
Catoptric systems do not have chromatic aberration at all and no secondary spectrum. A unit magnification high resolution catoptric system is described, for example, in U.S. Pat. No. 3,748,015. A two mirror system of this type is commercially available from the assignee of this patent, the Perkin-Elmer Corporation, but the N.A. is only about 0.167. This results in resolution capability sufficient to produce linewidths of only about 2.5 to 3.0 microns. The resolution capability of this system may be slightly improved but for linewidths below 2.0 microns the mechanical tolerances on motion, depth of focus and mirror stability and quality are beyond the state-of-art.
Unit magnification catadioptric systems have also been proposed. The simplest such system, a 1:1 flat field telescope, was discussed by Dyson in 1959 (49 J. Opt. Soc. Amer. 713). The simple Dyson system appears herein as FIG. 1 and consists of a plano-convex lens concentric with a spherical mirror. The plane surface of the lens intersects the center of curvature and is therefore imaged inverted onto itself. Dyson used one half of the plane surface to image onto the other half. In U.S. Pat. No. 3,536,380 a beam-splitting semi-reflecting surface has been added to enable the object and image planes to be separated without sacrificing field size. The Dyson system and various improvements thereon are described by C. G. Wynne in "A Unit-Power Telescope for Projection Copying", published by Oriel Press Limited in Optical Instruments and Techniques, pages 429-434, edited by J. H. Dickson (1970). Wynne suggests compounding the plano-convex lens into a onomeniscus and a plano-convex lens to correct aberrations in the system. By suitable choice of glasses and dispersions, Wynne notes that either the variation with frequency of Petzval sum or the Seidel spherical aberration can be corrected, but not both. So it was suggested that the Seidel spherical aberration not be fully corrected but instead balanced across the field against the departure from exact Petzval condition.
The inventors have discovered that the Wynne configuration cannot be corrected as well as believed. Even when more meniscus lenses are compounded to the front of the plano-convex lens to obtain more degrees of freedom and better correction, there is still an unacceptably large chromatic variation in astigmatism and Petzval sum. The more dominant need to correct Seidel spherical aberration seems to necessarily leave a chromatic variation in astigmatism and Petzval sum which cannot be well corrected by adding more degrees of freedom to the compound plano-convex lens structure in classical manner. It was surprisingly discovered that the source of the difficulty is in not correcting the lateral chromatic aberration. Presumably this aberration does not need to be corrected because the symmetry of the 1:1 telescope causes the lateral chromatic aberration introduced by the lens structure in the path going to the mirror to be exactly cancelled by the lateral chromatic aberration introduced by the lens structure in the path coming from the mirror. The mirror itself of course causes no chromatic aberration at all. However, even though the lateral chromatic aberration cancels itself in a symmetric system, it still results in a variation in path length with frequency, which leads to a chromatic variation in astigmatism and Petzval sum.